Growth behavior and structure of alkyltrichlorosilane monolayers bearing thioacetate and acetate tailgroups

The growth behavior of self-assembled monolayer (SAM) films formed from Cl₃SiC₁₁H₂₂OCOCH₃ (C₁₁Ac) and Cl₃SiC₁₆H₃₂SCOCH₃ (C₁₆Sc) on oxidized Si wafers at room temperature was investigated using atomic force microscopy (AFM), water contact angle measurements, and ellipsometry. Both silanes form thin films via nucleation of islands, similar to the growth behavior of n-octadecyltrichlorosilane (OTS) SAMs, although the island morphology is distinct for each molecule. Near-confluent monolayer films are obtained for both C₁₁Ac and C₁₆Sc after substrate immersion periods of 10-60 min. The ellipsometric thickness of both types of SAMs is less than that expected for a close-packed, all-trans ensemble of methylene chains aligned parallel to the substrate surface normal, suggesting a liquid crystalline-like structure in which methylene groups are present at the outer film surface. This model is supported by surface wettability data obtained on C₁₁Ac SAMs that were reduced in situ to generate hydroxy-terminated films. When the deposition time is greater than 1 h, C₁₁Ac forms highly disordered films of multilayer thickness that are much rougher than the films obtained at shorter deposition times. Multilayer formation is not observed with C₁₆Sc, although increased deposition times also result in adsorption of additional, submonolayer amounts of material.